merge two sorted list and swap pairs

Author: zhongn2

Merge Two Sorted Lists.cpp

@@ -0,0 +1,61 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode(int x) : val(x), next(NULL) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
+        if(!l1) return l2;
+        if(!l2) return l1;
+        ListNode dummy(-1);
+        ListNode *p = &dummy;
+        for(; l1 != NULL && l2 != NULL; p = p -> next){
+            if(l1 -> val < l2 -> val){
+                p -> next = l1;
+                l1 = l1 -> next;
+            }else{
+                p -> next = l2;
+                l2 = l2 -> next;
+            }
+        }
+        // it is possible that one of the linked list is empty
+        // we need to check which one is NULL
+        p -> next = l2 == NULL? l1:l2;
+        return dummy.next;
+    }
+};
+
+// subtle method
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode(int x) : val(x), next(NULL) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
+        if(!l1) return l2;
+        if(!l2) return l1;
+        ListNode dummy(-1);
+        ListNode *p = &dummy;
+        while(l1 && l2){
+            if(l1 -> val > l2 ->val){
+                p ->next = l2;
+                l2 = l2 -> next;
+            }else{
+                p ->next = l1;
+                l1 = l1 -> next;
+            }
+             p = p -> next;
+        }
+        p -> next = l1 == NULL? l2:l1;
+        return dummy.next;
+    }
+};

Swap Nodes in Pairs.cpp

@@ -0,0 +1,25 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode(int x) : val(x), next(NULL) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode *swapPairs(ListNode *head) {
+        if(!head) return head;
+        ListNode dummy(-1);
+        dummy.next = head;
+        ListNode *p = &dummy, *q = p -> next, *k = q -> next;
+        for(;k (the same as k != NULL); p = q, q = q -> next k = q ? q -> next : NULL){ // overflow
+            p -> next = k;
+            q -> next = k -> next;
+            k -> next = q;
+        }
+        return dummy.next;
+    }
+};
+// for loop definition
+// for( initialization; condition; increase);